The present invention relates to a binary recording medium, and more particularly, to a medium wherein each binary bit includes a pair of transitions at predetermined spatial positions.
In present day data-processing systems use is being more frequently made of magnetic disc memories by reason of their storage capacity and of the relatively short time which the magnetic read/write heads take to access information situated anywhere on the discs from the moment when they receive an instruction to access this information from the processing system.
It is known that magnetic discs carry information in coded form on concentric circular recording tracks having a width that does not exceed a few hundredths of a millimeter and which are formed on both faces of the discs. The tracks are identified by allotting them a serial number j (j being a whole number) between 0 and (N-1), N being the total number of recording tracks. A coded expression of a track serial number j is called an address. The codes most frequently used are binary codes.
When memories contain a large number of discs (for very high storage capacity) one face of a single disc is set aside to have recorded on it the addresses of the tracks on all the other faces of the discs, which tracks contain only data intended for processing by the said system.
On the other hand, in memories of smaller storage capacity, which contain only one or two discs, the addresses of the tracks on one face of a disc are recorded on this same face in such a way that a maximum amount of space is set aside for recording data, while a minimum amount of space is set aside for addresses and information required to servo-control the position of a magnetic head to the tracks. In current practice the information for servo-controlling the head position is recorded within a plurality of reference zones, at least equal in number of the number of tracks; the zones are uniformly distributed over the entire surface of the disc. In one type of known system, each reference zone is divided into a number of cells, each of which contains information required to servo-control the position of the heads over the discs.
It is known that to record a series of items of information on a magnetic disc there are created on each track of the disc a succession of small magnetic domains termed "elementary magnetic areas" which are distributed along the entire length of the track and in which the induced magnetism is successively of opposite values and is orientated in a direction parallel to the surface of the disc. In effect, magnetization senses of the areas are opposed. In each cell, the items of information for servo-controlling position are formed by magnetization sense changes which may be in one of two predetermined positions.
These changes in the magnetization sense are repeated identically from one cell to the next in such a way that the magnetization sense of the first elementary magnetic area in each cell is always the same, which makes it necessary for there to be within each cell a second change in the magnetization sense, termed an "adjusting change".
In current practice, these "adjusting changes" always occupy identical positions within each cell and therefore cannot be used as information; this has the disadvantage of requiring space on the magnetic disc that serves no purpose.